Nic Lewis and Judith Curry have a new paper out in Climate Dynamics and report that climate sensitivity is even lower than previously thought. There is a long but somewhat technical writeup at WUWT, so this is my attempt to explain it all in layman's terms.
There seem to be two main strands of innovation in the paper. Firstly Nic and Judy have used the estimates of ocean heat uptake and radiative forcings reported in the Fifth Assessment itself. Secondly, they have made their energy budget approach somewhat more sophisticated in order to deal with the twin problems of volcanos and natural cycles.
The energy budget method involves calculating how much heat the Earth should have absorbed between a starting period (typically back in the 19th century) and a more recent closing period. Then by analysing temperature changes and ocean heat uptake, you should be able to get a feel for how much the surface should be warming per doubling of carbon dioxide. The complication comes if one or both of the two periods is affected by natural cycles or volcanos, which will bias the results one way or another. The new approach seeks to minimise this effect by finding opening and closing periods that are well-matched in terms of volcanic activity and positions in the natural cycles (such as the AMO).
Anyway the result of all this work is that ECS and TCR seem to be slightly lower than Lewis and Crok reported in their GWPF report on the subject.